Total knee arthroplasty has been a very successful procedure for the treatment of various types of joint disease. See, for example, "The Effect of Trochlear Design on Patellofemoral Shear and Compressive Forces in Total Knee Arthroplasty", W. J. Petersilge, M.D. et al., Clinical Orthopaedics and Related Research, Number 309, pp 124-130. This article has described in detail the various types of patella prosthesis and their affects on the shear forces in knee studies. This reference is incorporated herein in total by reference.
In particular patella prosthesis having a domed shape contact surface and round outer perimeter have been in use since the early 1970's. The advantage of such prosthesis are the simple instrumentation for insertion, the implant is positionable on the patella bone based on anatomy of the patient and can be placed medial to the center of the bone to improve tracking, the device is insensitive to poor alignment and contact area is increased when used with a deep conforming trochlear groove in a femoral component. The increase of contact area in a trochlear groove reduces the contact stress as the force is the same, however, the area over which the force is spread is increased with the increased contact area.
However, such round shaped prosthesis have a relatively poor ability to cover the cut surface of the patella. That is, the surface of the patella that is resected during surgery in order admit the prosthesis.
Patellas of this type are shown in prior art FIG. 1A and FIG. 1B.
In the early 1980's the patella prosthesis was modified along an oval biconcave form as shown in prior art FIG. 2. This modification provided better bone coverage due to a more anatomical shape and the patella forces were transferred from the dome portion of the patella at flexion angles below 75.degree. to the peripheral concave surfaces at patella flexion angles above 75.degree.. The three peg attachment design on the surface of the patella increased the contact surface also preserved blood supply and bone in the patella.
It has now been found that by providing a patella prosthesis with an oval periphery and a domed contact surface, i.e., substantially consistent radius, that reduced contact stress as well as increased bone coverage of the oval could be provided.